1. Field of the Invention
The present invention relates to a valve timing adjusting device for changing valve opening-closing timing suitable for use in intake and exhaust valves of an internal combustion engine.
2. Description of Related Art
As a conventional valve timing adjusting device, there is a well known vane-type device in which a camshaft is driven through a timing pulley, a chain sprocket, etc. which turn synchronously with an engine crankshaft. The valve timing of at least any one of an intake valve and an exhaust valve is hydraulically controlled by a phase difference of relative rotation of the timing pulley, the chain sprocket, and the camshaft. Engine output and fuel consumption ratio are improved by adjusting the phase difference between the crankshaft and the camshaft to an optimum value in accordance with engine operating state.
In such a vane-type valve timing adjusting device using operation oil, when at least any one of the intake valve and the exhaust valve is actuated, the camshaft receives a load torque which varies between positive and negative loads. Therefore, when the operation oil is not sufficiently supplied during cranking of the engine, there might arise such a problem that a vane member oscillates with respect to a housing member containing the vane member, thereby hitting against the housing member to produce knocks. The positive load torque is applied in the retarding direction of the camshaft with respect to the crankshaft, and the negative load torque is added in the advancing direction of the camshaft with respect to the crankshaft. Average positive and negative load torques is added in the retarding direction of the camshaft with respect to the crankshaft.
There has been such a well known device that, in case of insufficient supply of operation oil to the valve timing adjusting device, occurrence of knocks is prevented by preventing the vane member from oscillating with respect to the housing member by fitting a stopper piston in a fitting hole formed in the housing member. Therefore, when the operation oil is sufficiently supplied, the stopper piston is moved by the oil pressure out of the housing member, thereby enabling the control of rotation of the vane member with respect to the housing member.
Here, it is possible to reduce a pumping loss of the engine for improving the fuel consumption ratio by retarding the intake valve closing timing over the BDC position of a piston. However, when the intake valve closing timing is retarded over the BDC position of the piston, the fuel consumption ratio is improved after an engine warm-up, but a real compression ratio becomes lower at the time of cold engine, so that the air temperature does not sufficiently rise at the top dead center (TDC) of the piston. Thus, the engine might fail in starting. In this case, an optimum valve timing of the intake valve during the period of engine cooling is at the advance side of an optimum valve timing after the engine warm-up.
Therefore, it is considered to start the engine with certainty by fitting the stopper pin in the fitting hole to stop the engine when the vane member is in an intermediate position between the most advanced angle and the most retarded angle with respect to the housing member, and then by starting the engine when the vane member is in the intermediate position. As the valve timing adjusting device described above are disclosed in JP-A-9-324613 and JP-A-11-343819.
Generally, when the engine is stopped, the oil pressure added to each oil pressure chamber drops, and the vane member is turned to the retard side with respect to the housing member by a load torque applied to the camshaft. Therefore, when the vane member is positioned at the advance side over the intermediate position with respect to the housing member, the vane member is rotated to the retard side by the load torque when the engine is stopped and reaches the intermediate position to allow the stopper piston to fit in the fitting hole.
However, when the vane member is at the advance side of the intermediate position with respect to the housing member, the engine might stop due to increased viscosity of the operation oil during a cold engine even when the load torque is applied to the camshaft while an engine does not operate. Even when the engine is stopped in such a condition that the vane member is at the advance side of the intermediate position with respect to the housing member, the load torque is applied to the camshaft during the engine cranking, and the vane member rotates to the retard side with respect to the housing member when the engine starts. Then, the stopper piston fits in the fitting hole, thereby starting the engine at the intermediate position.
However, when the engine is started up immediately after the engine stop, the oil pressure is added to the oil pressure chamber because the oil is filled in an oil passage. When the operation oil is supplied to the advance oil pressure chamber after the engine startup, the oil pressure in the advance oil pressure chamber increases before the vane member receiving the load torque turns to the retard side, thereby causing the vane member to be placed at the advance side of the intermediate position. In the case of the intake valve for example, when the engine is started up while the intake valve opening timing is advanced, the exhaust valve opening timing and the intake valve opening timing overlap each other, thereby failing in starting the engine up.
In the valve timing adjusting device disclosed in JP-A-11-343819, the operation oil is discharged out of the advance oil pressure chamber and the retard oil pressure chamber during engine startup, thereby allowing the vane member to rotate to the retard side at the time of engine startup.
However, since no operation oil is supplied to both the advance oil pressure chamber and the retard oil pressure chamber, sliding parts of members are not supplied with the operation oil at the time of engine startup, so that the sliding parts of members are likely to be seized up. Further, while no operation oil is supplied to both oil pressure chambers, when the stopper piston comes out of the fitting hole, the vane member is likely to turn to the advance side by the load torque, so go that the vane member hits against the housing member.
An object of the present invention is to provide a valve timing adjusting device in which a driven-side rotor is held at an intermediate position with respect to a driving-side rotor when the engine starts, for preventing seizure of sliding parts during engine startup operation and occurrence of knocks.
According to the valve timing adjusting device in the present invention, when the engine is stopped when the drive-side rotor is at an advance side of the intermediate position with respect to the driving-side rotor, the hydraulic fluid can be discharged from an advance chamber to a drain while supplying the hydraulic fluid from a fluid supply source to the advance chamber by simultaneously connecting the advance fluid passage to the fluid supply source, and the advance fluid passage to the drain, at the time of engine startup. Since the advance chamber fluid pressure remains low even when the hydraulic fluid is filled in the advance chamber, the driven-side rotor rotates to the retard side with respect to the driving-side rotor when the load torque is applied to the driven-side rotor at the time of engine startup. When the driven-side rotor reaches the intermediate position, a contacting portion contacts a contacted portion, thereby holding the driven-side rotor at the intermediate position with respect to the driving-side rotor. By setting the intermediate position at the optimum phase, the engine can be reliably started up. Upon engine starting up, the hydraulic fluid pressure rises to move the contacting portion away from the contacted portion, so that rotation of the driven-side rotor with respect to the driving-side rotor is controlled.
Since the hydraulic fluid can be discharged out of the advance chamber while supplying the hydraulic fluid into the advance chamber during engine startup, the hydraulic fluid circulates in the advance fluid passage and the advance chamber. Since the hydraulic fluid lubricates sliding parts of each member from just after the beginning of engine startup, it is possible to prevent seizure of the member at the time of engine startup.
Since the advance chamber is full of the hydraulic fluid, though at a low pressure, at the time of engine startup, the driven-side rotor is prevented from rotating to the retard side to hit against the driving-side rotor even when the contacting portion is released from the contacted portion.